I. Field of the Invention
The present invention relates to a gear tooth and to a pump, especially an oil pump equipped with corresponding gears.
More precisely, this invention has as its object a gear tooth provided with a root that is concave at its point of separation from the root of the neighboring tooth, and with a top joined to the said root.
This tooth is used preferably but not exclusively in an external gear pump provided with at least one pair of mutually meshed toothed pinions.
Such a pump, which is also the object of the invention, can be used in an internal combustion engine, but the invention is also applicable to all external gear pumps.
II. Discussion of the Background
The oil pumps used in engines are of two types: external gear pumps with straight or spherical involute teeth, and internal gear pumps, with straight trochoidal or spherical involute tooth profiles.
Modern generations of engines, and especially those of their accessories, place greater demands of oil flow and pressure on the pumps used. Moreover, the limits on space requirement within the engine environment are becoming increasingly tighter.
The conventional methods adopted to increase the hydraulic performances of gear pumps are in particular increase in the pump speed, increase in the height of the pump gears, reduction of the hydraulic backlash or increase in the number of pinions.
Nevertheless, oil pumps have low volumetric efficiencies at low speed, so that they are generally overdimensioned at high speed, and it is often necessary to discharge a large part—even as much as half—of the oil pumped at high speed via a discharge valve.
Different toothing profiles exist for external gear pumps. The standard geometry, of the straight spherical involute toothing type, has modest performances. In fact, any attempt to increase the volume of oil displaced by optimizing the tooth profile rapidly runs into problems of different constraints. The possibility of increasing the outside diameter of the tooth is limited by the small thickness thereof and by the risk of having an overly pointed tooth. In addition, elongation of the tooth results in a disadvantage for continuity of meshing, especially at the root of the tooth. Finally, the interference between the base circle and the root of the tooth also suffers from elongation thereof.
A traditional tooth profile for a gear pump comprises a trochoidal concave base followed by a spherical involute top.
It has already been proposed to improve the performances of an external gear pump by abandoning the spherical involute profiles in favor of other profiles such as epicycloids or hypocycloids joined to the primitive circle of the toothed gear, or in other words to the theoretical circular line that rolls over an equivalent line of the opposite tooth.
However, the gains achieved in this way compared with traditional toothings are insufficient. Moreover, by deviating therefrom, difficult technical choices and an increase in manufacturing costs are rapidly encountered.